Benzindene prostaglandins are known to be useful to treat a variety of conditions. Cyclopenta[b]naphthalenol derivatives of Formula 1
are key intermediates for the synthesis of benzindene prostaglandins. As shown in Scheme A, Aristoff et al. discloses in Tetrahedron Letters (1982), 23(20), 2067-2070 that a compound of Formula 1a can be obtained by sulfonylation of a compound of Formula 4a to form a compound of Formula 3a (Step a), removing the protecting group on the phenol group, i.e., tert-butyldimethylsilyl, for forming a compound of Formula 2a (Step b), and intramolecular alkylation of the compound of Formula 2a (Step c):

A similar process for the preparation of the cyclopenta[b]naphthalenol derivatives is also reported in U.S. Pat. No. 8,524,939. As shown in Scheme B, the compound of Formula 1b can be obtained by sulfonylation of a compound of Formula 4b (Step a) to form a compound of Formula 3b (Step a), removing the protecting group on the phenol group, i.e., benzyl, for forming a compound of Formula 2b (Step b), and intramolecular alkylation of the compound of Formula 2b:

In the aforementioned prior art procedures, in order to allow the sulfonyl group attaching to the primary alcohol, the sulfonylation reaction has to be carried out first and the protecting group on the phenyl group is removed subsequently. However, since the sulfonation products, i.e., the compounds of Formulae 3a and 3b, are unstable and have high activity, the processes suffer from disadvantages that while removing the protecting group on the phenol group, side reactions happen that cause the sulfonyl group to be detached from the primary alcohol or be replaced. Thus, the processes can only achieve a low yield, thus increasing the cost for removing side products.
In addition, the sulfonyl group of the compound of Formula 3b would undesirably affect the activity of the hydrogenation catalysts. Therefore, a large amount (such as about 30 to 40% w/w) of expensive hydrogenation catalysts has to be used in step b of Scheme B to allow the hydrogenation reaction for removing the benzyl group to be successfully carried out.
Consequently, there is a demand for processes for the preparation of the compound of Formula 1 that are commercially available of high yield and at a low cost and are more convenient to operate.